Noble metal-containing supported catalyst and a process for its preparation

ABSTRACT

The invention provides a noble metal-containing supported catalyst which contains one of the noble metals from the group Au, Ag, Pt, Pd, Rh, Ru, Ir, Os or alloys of one or more of these noble metals in the form of noble metal particles on a powdered support material. The particles deposited on the support material have a degree of crystallinity, determined by X-ray diffraction, of more than 2 and an average particle size between 2 and 10 nm. The high crystallinity and the small particle size of the noble metal particles lead to high catalytic activity for the catalyst. It is particularly suitable for use in fuel cells and for the treatment of exhaust gases from internal combustion engines.

This application is a divisional of U.S. patent application Ser. No.10/136,691, filed Apr. 30, 2002, now U.S. Pat. No. 6,861,387, the entiredisclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention provides a noble metal-containing supportedcatalyst which contains a noble metal selected from the group consistingof Au, Ag, Pt, Pd, Rh, Ru, Ir, Os and mixtures thereof and a process forthe preparation thereof.

BACKGROUND OF THE INVENTION

Noble metal-containing supported catalysts are used in many industrialfields such as, for example, the synthesis of chemical compounds, theconversion of harmful substances in the exhaust gases from internalcombustion engines and as electrocatalysts for fuel cells, to mentiononly a few fields of application.

To produce the highest possible catalytic activity for the noble metal,they have to be applied to the surface of the particular supportmaterial in the highest possible dispersion with particle sizes in therange between 1 and 15 nm. A small particle size in itself, however, isnot a guarantee of high activity. A poorly developed crystal structurein the platinum particles thus also leads to diminished catalyticactivity.

Similar considerations also apply to the quality of alloy formation ofalloy catalysts. It is known in the art that ternary alloy catalysts forfuel cells with an ordered crystal structure have a catalytic activityfor the electrochemical reduction of oxygen which is at least twice asgreat as that of a non-alloyed platinum catalyst. The catalyst isprepared by depositing the alloy components on the support material byimpregnation. The alloy is formed by thermal treatment at 900° C. for aperiod of one hour under an atmosphere of nitrogen.

Support materials which are used for supported catalysts include avariety of materials. In general, the support materials, depending onthe field of application, all have a high specific surface area, theso-called BET surface area (measured by nitrogen adsorption, inaccordance with DIN 66132), of more than 10 m²/g. For fuel cells,electrically conductive carbon materials are used as supports for thecatalytically active components. In the case of car exhaust catalysis,however, oxidic support materials such as, for example, active aluminiumoxides (for example γ-aluminium oxide), aluminium silicate, zeolite,titanium oxide, zirconium oxide, rare earth oxides or mixtures or mixedoxides thereof are used.

Precursor compounds of the catalytically active components are depositedon the surface of these materials and are converted into the finalcatalytically active form by subsequent thermal treatment. The finenessof distribution (dispersion) of the catalytically active particles inthe final catalyst, and thus the catalytic metal surface area availablefor the catalytic process, depends critically on the type of process andmethod used for these two processes (deposition and thermal treatment).

A variety of processes has been disclosed for deposition of thecatalytically active components on the powdered support material. Theseinclude, for example, impregnation with an excess of impregnationsolution. In this case an aqueous solution of the catalytically activecomponents is added to the powdered support material, when the volume ofthe solution may be substantially greater than the water absorptioncapacity of the support material. Thus a material is produced which hasa thick pasty consistency and which is dewatered, for example, in anoven at elevated temperatures of 80 to 150° C. Chromatographic effectsmay take place during the dewatering of this material which can lead tonon-uniform distribution of the catalytically active components on thesupport material.

For pore volume impregnation, an amount of solvent is used to dissolvethe catalytically active components which corresponds to about 70 to110% of the absorption capacity of the support material for thissolvent. The solvent is generally water. This solution is distributed asuniformly as possible, for example by spraying over the support materialwhich is being rolled about in a tank. After distribution of the entiresolution over the support material the latter is still free-flowing,despite the water content. Chromatographic effects can be largelyavoided using pore volume impregnation. This method usually providesbetter results than the impregnation process using an excess of solventdescribed above.

For a process for so-called homogeneous deposition from solution, thesupport material is first suspended in, for example, water. Then anaqueous solution of precursor compounds of the catalytically activecomponents is added using capillary injection with constant stirring.Capillary injection is understood to be the slow addition of thesolution under the surface of the suspension of support material, usinga capillary. As fast and as homogeneous a distribution as possible ofthe precursor compounds over the entire volume of the suspension isintended to be ensured by intensive stirring and slow addition. Here,some adsorption of the precursor compounds, and thus the formation ofcrystallisation seeds, takes place at the surface of the supportmaterial. The extent of this adsorption depends on the combination ofsupport material and precursor compound. With material combinationswhich do not ensure adequate adsorption of the precursor compounds onthe support material, or when chemical fixing of the catalyticallyactive components to the support material is desired, the precursorcompounds can be precipitated on the support material by capillaryinjection of a base into the suspension of the support material.

To complete preparation of the catalyst material, the support materialcoated with the catalytically active components is subjected to asubsequent thermal treatment which converts the precursors of thecatalytically active components into the catalytically active form andoptionally leads to the formation of an alloy. Temperatures of more than300° C. up to 1000° C. and treatment times of 0.5 to 3 hours arerequired for this. Typically, batch processes are used for this in whichthe catalyst material is agglomerated and the noble metal particlesbecome coarser due to the long treatment times and the sinter effectswhich take place. Noble metal particles up to 50 nm or larger candevelop in this way. To form an alloy, temperatures above 900° C. andtreatment times of at least 0.5 hours are usually required, whereinthere is a risk of excessive particle growth due to sintering.

However, it is important that the catalysts have as high a surface areaas possible (i.e. high dispersion) on the support in order to ensurehigh catalytic activity. Catalysts with average particle sizes for thenoble metals of more than 20 nm are usually not very active.

Support materials coated with catalysts using known processes fortreatment cannot simultaneously comply with the conflicting requirementsfor well developed crystallinity or alloy structure and small averageparticle diameters for the noble metal particles.

In an alternative process for the thermal treatment of powderedsubstances the powdered substances are treated in a high-temperatureflow reactor. The treatment temperature in the flow reactor may behigher than 1000° C. The time of treatment may be varied between 0.01seconds and a few minutes. Finely dispersed noble metals can then bedeposited on, for example, aluminium oxide.

It has also been suggested that a turbulent or laminar burner be used asan essential source of heat. The process is thus performed in anoxidizing atmosphere and is not suitable for preparing catalysts onsupport materials made of carbon (graphite, carbon black), such as thoseused for fuel cells. The carbon black support would be oxidized and somewould be burnt away.

Based on the forgoing, there is a need in the art for methods ofpreparing a noble metal-containing supported catalysts which have a highcrystallinity or a well-developed alloy structure. There is also a needfor noble metal-containing supported catalysts that have a smallparticle size and high dispersion.

SUMMARY OF THE INVENTION

The present invention provides a noble metal-containing supportedcatalyst which contains one of the noble metals from the group Au, Ag,Pt, Pd, Rh, Ru, Ir, Os or alloys of one or more of these metals on apowdered support material. The supported catalyst contains particles ofnoble metal deposited on the support material having a relative degreeof crystallinity C_(x), determined by X-ray diffraction, of more than 2,preferably more than 5, and an average particle size between 2 and 10nm.

For a better understanding of the present invention together with otherand further advantages and embodiments, reference is made to thefollowing description taken in conjunction with the examples, the scopeof the which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE FIGURE

The preferred embodiments of the invention have been chosen for purposesof illustration and description but are not intended to restrict thescope of the invention in any way. The preferred embodiments of certainaspects of the invention are shown in the accompanying figure, wherein:

FIG. 1 illustrates an apparatus for thermal treatment of the catalystprecursor to prepare the catalyst of the current invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in connection with preferredembodiments. These embodiments are presented to aid in an understandingof the present invention and are not intended to, and should not beconstrued, to limit the invention in any way. All alternatives,modifications and equivalents that may become obvious to those ofordinary skill upon reading the disclosure are included within the spritand scope of the present invention.

This disclosure is not a primer on preparing noble metal-containingsupported catalysts, basic concepts known to those skilled in the arthave not been set forth in detail.

The catalyst according to the invention, due to the thermal treatmentwhich is described below, has a very high crystallinity. The relativedegree of crystallinity C_(x), which can be determined by radiographicmeasurements, was introduced by the inventors for the quantitativedetermination of crystallinity. It is defined by equation (1):

$\begin{matrix}{C_{x} = \frac{I_{x} - I_{a}}{I_{a}}} & (1)\end{matrix}$The relative degree of crystallinity is determined by radiographicmeasurements on powdered samples (powder diffractometer from the StoeCo., copper Kα radiation). In equation (1) I_(x) represents theintensity of a specific diffraction reflex (measured in counts) from thecatalyst sample. In the case of platinum, for example, the (hkl111)-reflex is measured, which can be regarded as a measure of highelectrochemical activity for the reduction of oxygen. I_(a) is theintensity of X-ray diffraction from an X-ray-amorphous standard with thesame composition as the catalyst sample, wherein the intensity of theX-ray diffraction reflex from the sample is determined at the same angleas for the sample. In the case of a carbon-supported platinum sample,the amorphous standard is a material with a particle size for theplatinum of less than 2 nm which no longer exhibits any X-raydiffraction reflexes.

Depending on the intended application of the catalyst, different supportmaterials can be used. For use as anode or cathode catalysts in fuelcells, electrically conductive support materials based on carbon fromthe group carbon black, graphite, active carbon and fibrous, graphiticnanotubes are normally used. For car exhaust gas catalysts, on the otherhand, oxidic materials from the group of active aluminium oxide,aluminium silicate, zeolite, titanium oxide, zirconium oxide, rare earthoxides or mixtures or mixed oxides thereof are used. Furthermore, thenoble metals in the catalyst may also be alloyed with at least one basemetal from the group Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Thesebase metals act as promoters, that is they modify the catalytic effectof the noble metal.

The catalyst according to the invention is particularly preferablysuitable for use as an anode or cathode catalyst in fuel cells. As acathode catalyst it has, for example, platinum on carbon black in aconcentration between 5 and 80 wt. %, with respect to the total weightof support material and platinum. As an anode catalyst, on the otherhand, a CO-tolerant Pt/Ru alloy on carbon black in a concentrationbetween 5 and 80 wt. %, with respect to the total weight of supportmaterial and alloy, is used, wherein the atomic ratio of Pt to Ru isbetween 5:1 and 1:5. The support material, carbon black, intended forthese applications has a surface area of at least 40 m²/g.

An essential feature of the catalyst according to the invention is thatthe requirements for degree of crystallinity and for particle size aresatisfied simultaneously. It then exhibits superior properties when usedas a catalyst in fuel cells and for exhaust gas treatment for internalcombustion engines.

These requirements can be satisfied when the following steps are takenduring preparation. First, it has to be ensured that deposition of thenoble metal on the support material is performed in such a way that thenoble metal particles being formed are not larger than 10 nm. It wasfound that this condition can be complied with, for example, using porevolume impregnation or homogeneous deposition from solution. In the caseof homogeneous deposition from solution, the coated support material isseparated from the solution, dried and optionally subjected to mildcalcination, which is performed in such a way that no substantialincrease in the particle size of the noble metal particles occurs. Aprecursor of the catalyst which has to be subjected to further thermaltreatment in order to increase the crystallinity and optionally foralloy production is obtained in this way. In the case of pore volumeimpregnation, the impregnated material can be used directly as aprecursor for further thermal treatment without additional drying andcalcination steps.

Subsequent thermal treatment of the precursor of the catalyst mustensure that the requirements relating to relative degree ofcrystallinity and average particle size are complied with. It was foundthat this is possible when the precursor of the catalyst is subjected toa brief thermal treatment at temperatures between 1000 and 1800° C. fora period of less than one minute.

The heat energy required for thermal treatment should preferably betransferred to the support material by radiation. This procedure enablesrapid heating of the particles in the support material. Radiationheating is particularly preferred in the case of carbon-containingsupport materials such as, for example, carbon black or active carbon.These materials absorb the incident heat radiation almost completely andthus heat up particularly rapidly.

To perform thermal treatment of the support material, it is firstcontinuously dispersed in an inert carrier gas heated to a temperaturebetween 300 and 500° C. Preheating the carrier gas has to be restrictedto a temperature at which no substantial increase in the size of thenoble metal particles takes place. Then the gas stream is passed througha reaction tube. The temperature of the tube wall is maintained at thedesired treatment temperature of 1000 to 1800° C. by an external heatingsystem. The volume flow of the carrier gas is chosen so that theduration of passage through the reaction tube is in the range from a fewseconds up to at most one minute. This residence time is kept short sothat the actual heating of the support material takes place as a resultof the transfer of radiated heat and only to a small extent by thermalconduction from the tube wall via the carrier gas. Suitable residencetimes, also called treatment times in the following, may amount up to 1minute, but preferably are selected between 0.1 and 20 seconds and mostpreferably between 0.5 and 10 seconds.

Heating of the particles of support material by the supply of radiatedheat takes place substantially more rapidly than would be possible bythe transport of heat through the carrier gas. After leaving thereaction tube, the support material and carrier gas are cooled rapidlyto a temperature below about 500° C. in order to prevent excessivecrystallite growth. Afterwards, the catalyst material prepared in thisway is separated from the carrier gas stream and taken for subsequentuse.

Due to the very sudden heating up to the treatment temperature of thecatalyst precursor followed by cooling after only a very short treatmenttime, it is ensured that good crystallinity or alloy structure candevelop within the noble metal particles, but excessive particle growthdue to diffusion on the surface of the support material is suppressed.The short treatment times mean that the use of substantially highertreatment temperatures than those used for conventional calcination ispossible. The high treatment temperatures act in an advantageous manneron the speed with which the crystal structure of the noble metalparticles is developed.

The figure shows the main layout of a possible apparatus for thermaltreatment of the catalyst precursor in order to prepare a catalyst inaccordance with the invention. The catalyst precursor is the startingmaterial (1) and is supplied continuously to a gas disperser (2). Todisperse the powdered starting material, the disperser is provided withan inert dispersing gas (3), generally nitrogen. After leaving thedisperser, the dispersing gas loaded with starting material is admixedwith a so-called carrier gas (6) which has been heated in heating unit(7), before the mixing process, to an extent such that the temperatureof the solids/gas dispersion after mixing is between about 350 and 500°C. At this temperature, the solids/gas dispersion enters a reaction tube(4) which is heated from outside by a heating device (5) to the desiredtreatment temperature between 1000 and 1800° C. The volume flow of thecarrier gas added is such that the desired treatment time for thestarting material is obtained inside the reaction tube, taking intoaccount the dimensions of the reaction tube. After leaving the reactiontube, the carrier gas stream and the starting material enter a rapidcooling unit (8) in which the treated starting material is very rapidlycooled to a temperature of less than about 500° C. by blowing in, forexample, nitrogen (9). Finally, in the filter unit (10), the finalcatalyst material is separated form the carrier gas and is discharged asproduct (11).

Due to the short residence time of the starting material in the reactiontube, there is only a small transfer of heat due to thermal conductionvia the gas phase. Rather, the starting material is mainly heated veryrapidly by radiated heat from the wall of the reaction tube andaccordingly can also be cooled again very rapidly. To avoid theintroduction of air, a slight overpressure is maintained inside theentire apparatus.

As a result of the short-term thermal treatment described, the particlesizes of the noble metal particles are enlarged only very slightly. Dueto thermal treatment in conventional rotary kilns, or batchwise inchamber kilns, such short treatment times as those achieved with theapparatus described cannot be realised. In addition, in comparison toconventional thermal treatments in which the goods to be treated areintroduced in dishes, vats or other containers, there is substantiallyless agglomeration and caking of the catalyst material. This is achievedby dispersing the catalyst in a continuous stream of carrier gas.

Catalysts according to the invention have only small average particlesizes of less than 15 nm, preferably less than 10 nm, due to the specialthermal treatment process. Their specific metal surface area is in therange 20 to 200 m²/g. At the same time, they have a high crystallinity.As shown by determining the relative degree of crystallinity C_(x)defined above, this is a factor of 2, and in general even a factor of 5,greater than the relative degree of crystallinity of traditionalcatalysts.

A preferred area of application of the catalyst according to theinvention is its use as anode or cathode catalyst in fuel cells. In PEMfuel cells (polymer electrolyte membrane fuel cells), platinum andplatinum alloys on a conductive support material (mostly carbon black orgraphite) are used as anode and cathode catalyst. The concentration ofnoble metal is between 10 and 80 wt. %, with respect to the total weightof catalyst.

For the anode side of PEM fuel cells (polymer electrolyte membrane fuelcells), carbon black supported platinum/ruthenium catalysts aregenerally used. The ratio platinum/ruthenium is in the range Pt/Ru=5:1to 1:5 (atomic ratio), wherein the ruthenium, in an electrochemicalRedox reaction with water (“spill over effect”), reduces CO-poisoning ofthe platinum catalyst. Carbon monoxide-containing hydrogen mixtures areused in the case of reformate-operated fuel cells.

PtRu electrocatalysts have long been known in the prior art relating tothis area. To condition the materials for PtRu electrocatalysts, costlybatch processes are used in which the size of the catalyst particles isincreased.

For the cathode side of PEM fuel cells, pure Pt catalysts with a Ptloading of 20 to 80 wt. % are preferably used. However, alloys ofplatinum with base metals (BM) such as chromium, tungsten, nickel,copper or cobalt are also used. The amounts added here are generally inthe range Pt/BM=5:1 to 1:5 (atomic ratio).

With an anode catalyst according to the invention, based on PtRu/C, thehigh crystallinity brings about reduced adsorption of carbon monoxide onthe crystallite surface and thus a reduced tendency to be poisoned. Thecatalyst thus has a higher tolerance towards carbon monoxide.

On the cathode side of fuel cells, where pure platinum catalysts areused, the activity of the catalyst for oxygen reduction reaction (ORR)is determined by the number of crystallite planes in the platinumcrystal. In order to increase the activity of Pt electrocatalysts,therefore, it is insufficient simply to maximise the Pt surface area.Rather, it is necessary to achieve high crystallinity with large Ptsurface areas in order to maximise the fraction of (100), (110) and(111) platinum surface atoms in proportion to the total number ofplatinum atoms. This requirement is complied with in an ideal manner bythe catalyst according to the invention. Therefore it is especiallysuitable for use in low-temperature fuel cells (PEMFC, DMFC, PAFC).

Having now generally described the invention, the same may be morereadily understood through the following reference to the followingexamples, which are provided by way of illustration and are not intendedto limit the present invention unless specified.

EXAMPLES

The following examples are intended to explain the invention further.

Example 1 Anode Catalyst for PEM Fuel Cells

Two kilograms of a carbon black supported electrocatalyst (noble metalloading 26.4 wt. % platinum and 13.6 wt. % ruthenium on Vulcan XC 72,atomic ratio Pt:Ru=1:1, prepared in accordance with U.S. Pat. No.6,007,934) are metered into a gas disperser using a dosing balance andfinely distributed with nitrogen as dispersing gas. The catalyst is thentransported into the reaction tube in a stream of nitrogen preheated to350° C.

Process Parameters:

Carrier gas: nitrogen Amount of carrier gas: 8 m³/hour (nitrogen)Temperature (carrier gas):  350° C. Treatment temperature: 1300° C.Treatment time: 3 s (approx) Amount metered in: 1100 g/hour

The treated catalyst is cooled with nitrogen in the rapid cooling unitand collected in the filter unit. A process control system is used toadjust the parameters and to monitor the same.

The catalyst treated in this way has the following properties:

Radiographic Measurements (Reflection hkl 111, 2 Theta ca. 40°):

Particle size (XRD):   6.3 nm Lattice constant: 0.3852 nm Intensity(I_(x), XRD):   2800 counts Intensity (I_(a), XRD)   400 counts Degreeof crystallinity C_(x):    6For comparison, the untreated starting material has the following data:

Particle size (XRD):   2.6 nm Lattice constant: 0.3919 nm Intensity(I_(x), XRD):   800 counts Intensity (I_(a), XRD)   400 counts Degree ofcrystallinity C_(x):    1

Due to the high crystallinity and, at the same time, small particlesize, the treated electrocatalyst exhibits very good electricalproperties in a PEM fuel cell, in fact as an anode catalyst under bothhydrogen/air and also reformate/air operation.

Comparison of Example 1 PtRu/C with Conventional Thermal Treatment

100 grams of the carbon black-supported electrocatalyst (noble metalloading 26.4 wt. % platinum and 13.6 wt. % ruthenium on Vulcan XC 72,atomic ratio Pt:Ru=1:1, compare with example 1) are treated at 850° C.for 60 min under nitrogen in a conventional batch process. After thermaltreatment in the kiln the material is allowed to cool under a protectivegas.

Properties:

Particle size (XRD):  13.6 nm Lattice constant: 0.3844 nm Intensity(I_(x), XRD):   1300 counts Intensity (I_(a), XRD)   400 counts Degreeof crystallinity C_(x):  2.25

In direct contrast to example 1, the catalyst has a lower performance inPEM fuel cells due to the high particle size of 13.6 nm.

Example 2 Pt/C Supported Catalyst for PEM Fuel Cells

One kilogram of a carbon black-supported electrocatalyst (platinumloading 40 wt. % on Vulcan XC 72) is metered into a gas disperser usinga dosing balance and finely distributed with nitrogen as the injectorgas stream. The catalyst is then transported into the reaction tube in astream of nitrogen preheated to 350° C. Process parameters:

Carrier gas: nitrogen Amount of carrier gas: 8 m³/hour (nitrogen)Temperature (carrier gas):  350° C. Treatment temperature: 1200° C.Treatment time: 3 s (approx) Amount metered in: 1000 g/hour

The treated catalyst is cooled with nitrogen in the rapid cooling unitand collected in the filter unit. A process control system is used toadjust the parameters and to monitor the same.

The catalyst treated in this way has the following properties:

Particle size (XRD):   6.5 nm Lattice constant: 0.3931 nm Intensity(I_(x), XRD):   3000 counts Intensity (I_(a), XRD)   400 counts Degreeof crystallinity C_(x):   6.5For comparison the untreated starting material has the following data:

Particle size (XRD):   3.9 nm Lattice constant: 0.3937 nm Intensity(I_(x), XRD):   1600 counts Intensity (I_(a), XRD)   400 counts Degreeof crystallinity C_(x):    3

Due to the high degree of crystallinity and, at the same time, smallparticle size, the treated electrocatalyst exhibits very good electricalproperties in a PEM fuel cell, in fact in particular as a cathodecatalyst under hydrogen/air operation.

Example 3 PtCr/C Alloy Catalyst for PEM Fuel Cells

One kilogram of a carbon black-supported electrocatalyst (platinumcontent 40 wt. % on Vulcan XC 72, atomic ratio Pt:Cr=3:1) are meteredinto a gas disperser using a dosing balance and finely distributed withnitrogen as dispersing gas. The catalyst is then transported into thereaction tube in a stream of nitrogen preheated to 350° C.

Process Parameters:

Carrier gas: nitrogen Amount of carrier gas: 8 m³/hour (nitrogen)Temperature (carrier gas):  350° C. Treatment temperature: 1400° C.Treatment time: 3 s (approx) Amount metered in: 1000 g/hour

The treated catalyst is cooled with nitrogen in the rapid cooling unitand collected in the filter unit. A process control system is used toadjust the parameters and to monitor the same.

The catalyst treated in this way has the following properties:

Radiographic measurements (Reflection hkl 111, 2 Theta ca. 40°):

Particle size (XRD):  7.5 nm Lattice constant: 0.385 nm Intensity(I_(x), XRD):  3200 counts Intensity (I_(a), XRD)   400 counts Degree ofcrystallinity C_(x):    7

Due to the high degree of crystallinity and, at the same time, smallparticle size, the treated electrocatalyst exhibits very good electricalproperties in a PEM fuel cell, in fact in particular as a cathodecatalyst under hydrogen/air operation.

Comparison of Example 2 PtCr/C with Conventional Thermal Treatment

100 grams of a carbon black-supported electrocatalyst (platinum content40 wt. % platinum on Vulcan XC 72, atomic ratio Pt:Cr=3:1, compare withexample 3) are treated under forming gas at 900° C. for 60 min in aconventional batch process. After thermal treatment in the kiln, thematerial is allowed to cool under a protective gas.

Properties:

Particle size (XRD):   16 nm Lattice constant: 0.386 nm Intensity(I_(x), XRD):  2000 counts Intensity (I_(a), XRD)   400 counts Degree ofcrystallinity C_(x):    4

In direct comparison to example 4, the catalyst, has a low performancein PEM fuel cells due to the high particle size of 16 nm.

Example 4 Pt/Aluminium Oxide Catalyst for Gas Phase Catalysis

Ca. 2 kg of a moist powder, prepared by pore volume impregnation of thesupport with the noble metal solution (incipient wetness method)consisting of:

78 wt. % aluminium oxide (γ-Al₂O₃, BET surface area 140 m²/g) 20 wt. %water  2 wt. % platinum nitrateare metered into the gas disperser using a dosing balance, finelydistributed with nitrogen as dispersing gas and transported into thereaction tube.Process Parameters:

Carrier gas: nitrogen Amount of carrier gas: 8 m³/hour (nitrogen)Temperature (carrier gas):  350° C. Treatment temperature: 1100° C.Treatment time: 3 s (approx) Amount metered in: 1000 g/hour

After leaving the reaction tube, the treated catalyst is cooled withnitrogen in the rapid cooling unit and collected in the filter unit. Aprocess control system is used to adjust the parameters and to monitorthe same.

The catalyst treated in this way has the following properties:

Composition: 2.5 wt. % Pt on aluminium oxide Particle size (XRD): 5 nmIntensity(I_(x), XRD): 3400 counts Intensity (I_(a), XRD)  400 countsDegree of crystallinity C_(x): 7.5

The catalyst treated in a conventional process (900° C., residence time60 min, nitrogen), on the other hand, has a particle size of 12 nm and adegree of crystallinity C_(x)=4.

The catalyst from example 4 is used in gas phase catalysis, for exampleas a catalyst for the treatment of exhaust gases from internalcombustion engines or as a catalyst for the selective oxidation of CO inso-called PROX reactors for the purification of hydrogen in fuel cellsystems.

Due to the small particle size and, at the same time, highcrystallinity, very good results are obtained, in particular for theworking life/durability of the catalyst.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departure from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth and as follows in the scope ofthe appended claims.

1. A noble metal-containing supported catalyst comprising one or morenoble metals selected from the group consisting of Au, Ag, Pt, Pd, Rh,Ru, Ir, Os and alloys thereof deposited in the form of noble metalparticles on a powdered support material, wherein the noble metalparticles have a relative degree of crystallinity of greater than 2 asdetermined by X-ray diffraction and an average particle size of betweenabout 2 and about 10 nm.
 2. A supported catalyst according to claim 1,wherein the relative degree of crystallinity as determined by X-raydiffraction is greater than
 5. 3. A supported catalyst according toclaim 1, wherein the support material is a carbon-containing materialselected from the group consisting of carbon black, graphite, activecarbon and fibrous, and graphitic nanotubes.
 4. A supported catalystaccording to claim 3, wherein the noble metals are alloyed with at leastone base metal selected from the group consisting of Ti, Zr, V, Cr, Mn,Fe, Co, Ni, Cu and Zn.
 5. A supported catalyst according to claim 3,wherein the supported catalyst contains Pt on carbon black with asurface area of at least 40 m²/g in a concentration between 5 and 80 wt.% based on the total weight of support material and Pt.
 6. A supportedcatalyst according to claim 3, wherein the supported catalyst contains aPt/Ru alloy on carbon black with a surface area of at least 40 m²/g in aconcentration between 5 and 80 wt. % based on the total weight ofsupport material and alloy, and wherein the atomic ratio Pt to Ru isbetween 5:1 and 1:5.
 7. A supported catalyst according to claim 1,wherein the support material is an oxidic material selected from thegroup consisting of active aluminum oxide, aluminum silicate, zeolite,titanium oxide, zirconium oxide, rare earth oxides, and mixturesthereof.
 8. An anode or cathode catalyst for a low-temperature fuel cellcomprising the catalyst of claim
 1. 9. A catalyst for the treatment ofexhaust gases from an internal combustion engine comprising the catalystof claim
 1. 10. A process for preparing a supported catalyst accordingto claim 1, comprising providing a support material coated with aprecursor of the noble metals by pore volume impregnation; drying thesupport material; and thermally treating the dried support material at atemperature between 1000 and 1800° C. for a period of less than oneminute, wherein crystallinity is developed.
 11. A process for preparinga supported catalyst according to claim 1, comprising providing asupport material coated with a precursor of the noble metals byhomogeneous deposition from solution; drying the support material; andthermally treating the dried support material at a temperature between1000 and 1800° C. for a period of less than one minute, whereincrystallinity is developed.
 12. A process according to claim 11, whereinheat energy required for thermal treatment is transferred to the supportmaterial by radiation.
 13. A process according to claim 11, wherein thesupport material coated with the noble metal is continuously dispersedin an inert carrier gas stream at a temperature between about 300 and500° C., passed through a heated reactor and, after leaving the reactor,is rapidly cooled and then separated from the carrier gas stream.
 14. Aprocess according to claim 13, wherein the inert gas stream and supportmaterial are cooled to a temperature below 500° C. by admixing an inertand cooled gas or mixture of gases with the carrier gas stream.
 15. Aprocess for preparing a supported catalyst according to claim 1comprising providing a precursor of the supported catalyst, theprecursor having one or more catalytically active noble metals with anaverage particle size between 1 and 10 nm, on a surface of the supportmaterial, and thermally treating the precursor at a temperature betweenabout 1000 and 1800° C. for a period of less than one minute.